GEARBOX PROBLEM

1996 SAAB 900 • 160 miles

A month before, I drove my car through the flooding area(on road) and the level of water was about 40 cm and for the distance of about 500 m. I drove passed that way for about 4 or 5 times.
I saw the colour of gear oil had changed to pink that meant some water had filtered through the gearbox. Then I changed the gear oil, only one time is not enough, I totally changed the oil for 6 times to ensure the water will not left inside.
The problem now is that after left from the starting point, it sometimes, the "check gearbox" lights up and then my car lose all the power, so I must restart the engine then the "check gearbox" will distinguish and I can drive my car normally. Still now this problem is happened for sometimes, for the lucky trip this would not occur.
For my opinion, because I am only one person to drive this car, I guess everything inside the gearbox still in the good status but I do not know what or where that the water had influential effect.
Or the cause is that when sometimes if the gear position is on "P(park)" position I cannot start the engine(not so often). The gear lever is not become loose. And I had read in 2carpros. Com to fix this is by cleaning the neutral safety switch or change if necessary or the change the plastic part at gear lever.
Please give me the right solution. Thank you very much.